This invention relates to the manufacture of golf club heads, specifically to the manufacture of golf club heads having a titanium striking surface.
Golf clubs commonly known as xe2x80x9cwoodsxe2x80x9d traditionally have a head made of a suitable wooden material such as maple or persimmon. These wooden club heads are usually solid and are shaped with their weight properly distributed about their center of gravity to maximize performance. More recently, so-called metal xe2x80x9cwoodxe2x80x9d heads have been developed. These metal woods are formed of suitable metals such as steel or aluminum and, more recently, titanium. Metal wood clubs typically comprise a hollow shell having relatively thin walls including a thin front wall which is used to impact the golf ball. The pressure to obtain higher and higher performance out of these metal wood clubs has resulted in clubs designed with larger and larger head sizes and, in order to maintain proper swing weight, thinner and thinner walls.
Titanium has become the material of choice for ultra high performance metal woods. Titanium alloys usually have a modulus of elasticity (stiffness) that is lower than steel, but much higher than aluminum or magnesium. In comparison to aluminum and steel alloys, titanium alloys have a thirty percent or greater strength to weight ratio. Thus, manufacturing a metal wood from titanium gives the designers the ability to make a larger club head without compromise in strength or weight. Titanium, however, is not as easily manufactured and processed as steel or aluminum. High temperature processes such as casting, forging or heat treating must be carefully set up and controlled to prevent embrittlement that leads to rapid failure of the club head in service. Most titanium alloys are notch sensitive. Notch sensitivity means that tensile stress applied along a sharp inside corner will easily produce a crack that will propagate through the part resulting in a catastrophic failure.
Pure titanium appears in two forms. At room temperature a pure piece of titanium is a form where all of its atoms are arranged in a hexagonal close packed crystal structure. The hexagonal close packed crystal is called the xe2x80x9calpha phasexe2x80x9d of titanium. If the alpha phase of titanium is heated to above 1620xc2x0 F. the atoms rearrange from the hexagonal prism into a cubic pattern known as a xe2x80x9cbody centered cubicxe2x80x9d crystal structure. This phase of titanium is referred to as the xe2x80x9cbeta phase.xe2x80x9d
When titanium is heated (e.g. during casting or welding) oxygen can dissolve into the titanium resulting in a dense, stable alpha phase on the surface of the golf club head. The oxygen stabilized alpha phase formed on the surface is commonly called the xe2x80x9calpha case.xe2x80x9d The oxygen-stabilized alpha case is strong and exceptionally hard, however, it is also very brittle
Heretofore state of the art titanium drivers have been manufactured and sold with the alpha case in place. Indeed metalagraphic examination of three leading titanium drivers indicate that the alpha case left on the front wall of state of the art titanium drivers is from 0.001 to 0.010 inches thick on each side of the wall. Face cracking of titanium drivers is the predominant failure mode of these clubs and has led to numerous designs for reinforcing the front wall using ribs, cantilevered supports, and exotic variable wall thickness designs. Nevertheless, face cracking still constitutes a serious problem for the designs of state of the art titanium clubs. The inventor of the present invention determined that although the alpha case is quite hard and durable, and therefore intuitively a good club surface, the brittleness of alpha case makes it a potential point source for crack propagation, which, due to the inherent notch sensitivity of titanium could lead to rapid failure in service, especially in high impact environments.
The inventor of the present invention has determined that where a golf club head is manufactured with a titanium striking surface, the golf club could be made stronger by actually removing the alpha case even though the resulting wall thickness of the striking surface would actually be thinner than the wall thickness of the club with the alpha case. This is because, although the front wall of the club is thinner, the resulting striking surface is more ductile, and less crack prone, than a thicker wall surface with the alpha case intact. The inventor of the present invention also determined that by removing the alpha case (without removing the underlying ductile titanium alloy) enough weight is saved in the casting that a separate weight member could be attached to the sole plate of the finished club to permit fine tuning of the center of mass and polar moment of inertia of the finished club.
The present invention comprises a method of making a golf club head comprising forming the body of a golf club head by casting a molten titanium alloy in a mold and allowing the molten titanium to solidify. The titanium will unavoidably form an alpha case, which may typically be from 0.001-0.010 inch in thickness on a front wall that may be from 0.040 inches in thickness at its thinnest point to 0.180 inches in thickness at its thickest point and on the club body which may be as thin as 0.030 inch in certain locations. The alpha case is then removed by a conformal milling process, which uniformly dissolves the alpha case without distorting the underlying metal. According to one embodiment of the present invention, the alpha case is removed by a chemical milling process in which the casting is immersed in an aquious solution of hydrofluoric acid (HF) hydrofluoric/nitric acid (HFxe2x80x94HNO3); hydrofluoric/chromic acid (HFxe2x80x94CrO3) or similar acid solutions. The chemical milling process removes the alpha case uniformly over the entire surface of the part resulting in thinner wall sections but no significant distortion of the part from its as-cast profile. According to another embodiment of the present invention, the alpha case is removed by a plasma milling process in which the part is exposed to a gas such as carbon tetrafluoride (CF4), sulfur hexafluoride (SF6) or other halide gas in a high vacuum environment at elevated temperature. As with chemical milling, plasma milling uniformly removes the alpha case, resulting in thinner cross sections but no significant distortion of the part from its as-cast profile. Additionally, plasma milling has the added advantages of being selective for the oxygen embrittled alpha case and not the pure titanium alloy substrate and further, provided the gaseous mixture does not contain hydrogen, will not cause hydrogen embrittlement of the underlying substrate.
Irrespective of the method by which the alpha case is removed, the reduction in wall thickness results in a concomitant reduction in the weight of the part without any loss in the critical impact strength of the part (in fact, impact strength is increased). The reduction in the weight of the part can then be redistributed as a supplemental weight member, which can be attached to the sole plate of the club or cast as an integral part of the sole plate or as part of the lower surface of the body of the club, thereby lowering the center of mass of the club. The weight member can further be positioned on the sole plate or club body in such a way so as to permit fine tuning of the location of the center of mass of the club, as well as shaped so as to increase the polar moment of inertia of the club head about the golf club shaft. Thus, the combination of chemical milling of the golf club head and redistribution of the weight saved by removing the alpha case results in a more durable high performance golf club head having an increased polar moment of inertia for stability as well as a lower center of mass that can be adjusted to provide an adjustable launch angle.